A current sensing resistor, when serially connected to a load and applied current thereto, results in a voltage drop which may be measured and referred to estimate the current intensity. Since the resistance of a current sensing resistor is generally at a milliohm (mOhm) order, high resistance precision, e.g. with deviation within ±1%, is required compared to a common resistor. Accordingly, proper adjustment is generally performed in the manufacturing process of the current sensing resistor after measuring resistance of the newly produced resistor and calculating deviation of the measured resistance from a preset ideal value. Repetitive measurement and adjustment are performed until the measured resistance is close enough to the preset ideal value.
Conventionally, Kelvin measurement, which is a four-point type of measurement, is adopted to measure resistance of a current sensing resistor. The principle will be described hereinafter.
Please refer to FIG. 1, which schematically illustrates circuitry associated with Kelvin measurement. As shown, two ends of a resistor 15 whose resistance R is to be measured are respectively connected to four points 11, 12, 13 and 14. The points 13 and 14 are further respectively connected to head and tail ends of a constant current source 16 which supplies a constant current intensity I. On the other hand, the points 11 and 12 are coupled to respective probes with high impedance for measuring voltage difference therebetween. Since the input impedance of the probes coupled to the points 11 and 12 is relative high, it is assumed that no current would pass through point 11, resistor 15 and point 12, i.e. i1=0, i2=0. Under this circumstance, the constant current source 16, point 14, resistor 15 and point 13 form a circuit loop, and the voltage difference V between the points 11 and 12, where V=V11−V12, can be measured and used for calculating resistance of the resistor 15 based on Ohm's Law, i.e. V=IR.
FIG. 2A illustrates a structure of a conventional current sensing resistor as described in U.S. patent application Ser. No. RE39,660E, which is incorporated herein for reference. The current sensing resistor 100 includes a resistor plate 120 and two electrode plates 110 and 130 respectively welded to opposite sides of the resistor plate 120 and having apertures 140 and 150. On the electrode plates, sensing pads 111 and 113 and current pads 112 and 132 are defined as measuring area. When producing the current sensing resistor 100, a constant current I is applied between the current pads 112 and 132, and a voltage difference rendered between the sensing pads 111 and 131 (Vdiff=V111−V131) when the constant current I passes through the current sensing resistor 100 is measured. Accordingly, resistance R1 of the resistor 120 can be calculated as R1=Vdiff/I.
Please refer to FIG. 2B, which illustrates four measurement points defined in a measuring apparatus for measuring resistance of a newly produced resistor. The four measurement points 211, 212, 213 and 214 are arranged as a rectangle, wherein the measurement points 213 and 214 are associated with constant current input and the measurement points 211 and 212 are associated with output voltage measurement. The four measurement points 211, 212, 213 and 214 are substantially a constant distance from a resistor to be measured.
If measurement is conducted before a resistor belt is physically divided into resistor plates, the measurement points may be inconsistent for different plates due to mechanical deviation. For example, as shown in FIG. 2C and FIG. 2D, it may occur that the four measurement points are located at positions 311, 312, 313 and 314 (FIG. 2C) on a plate but located at different relative positions 311a, 312a, 313a and 314a on another plate (FIG. 2D).
Aside from, even if measurement is conducted twice for the same plate, deviation may also occur. For example, the four measurement points are located at positions 411, 412, 413 and 414 on the plate this time but located at different relative positions 411a, 412a, 413a and 414a on the plate next time, as illustrated in FIG. 2E. Assume a resistor 400 with desired resistance R is to be produced. During the production of the resistor 400, first measurement is performed and the four measurement points are located at the positions 411, 412, 413 and 414 on the plate so as to acquire a first resistance R1. If the first resistance R1 is not close to the desired R to a required extent, the different R-R1 needs to be offset and then second measurement is performed. Generally, it is expected that the second measurement would render a resistance closer to the desired resistance R than the first resistance R1. However, if the second measurement is performed at different relative positions 411a, 412a, 413a and 414a on the plate 400, the first measurement becomes non-referable for the improvement of the second measurement. Instead, a second resistance R2 which is still not close enough to the desired resistance R may be acquired. Such a mechanic misalignment problem occurring in the automation process is thus detrimental to Kelvin measurement. It is critical to minimize such deviation resulting from misalignment.